A&A 447, 465–472 (2006) Astronomy DOI: 10.1051/0004-6361:20053819 & c ESO 2006 Astrophysics

Large-scale magnetized outflows from the Cluster spiral NGC 4569

A galactic wind in a ram pressure wind

K. T. Chy˙zy1, M. Soida1,D.J.Bomans2, B. Vollmer3,Ch.Balkowski4, R. Beck5, and M. Urbanik1

1 Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland e-mail: [email protected] 2 Astronomisches Institut, Ruhr-Universität-Bochum, 44780 Bochum, Germany 3 CDS, Observatoire astronomique de Strasbourg, UMR 7550, 11 rue de l’Université, 67000 Strasbourg, France 4 Observatoire de Paris, GEPI, CNRS UMR 8111, and Université Paris 7, 5 Place Jules Janssen, 92195 Meudon Cedex, France 5 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany

Received 12 July 2005 / Accepted 3 October 2005

ABSTRACT

Using the Effelsberg radio telescope at 4.85 GHz and 8.35 GHz we discovered large symmetric lobes of polarized radio emission around the strongly Hi deficient Virgo cluster spiral NGC 4569. These lobes extend up to 24 kpc from the galactic disk. Our observations were complemented by 1.4 GHz continuum emission from existing Hi observations. This is the first time that such huge radio continuum lobes are observed in a cluster . The eastern lobe seems detached and has a flat spectrum typical of in-situ cosmic ray electron acceleration. The western lobe is diffuse and possesses vertical magnetic fields over its whole volume. The lobes are not powered by an AGN, but probably by a nuclear starburst that occurred ∼30 Myr ago, producing ≥105 supernovae. Since the radio lobes are symmetric, they resist ram pressure due to the galaxy’s rapid motion within the intracluster medium.

Key words. : individual: NGC 4569, IC 3583 – galaxies: magnetic fields – radio continuum: galaxies

1. Introduction pressure stripping event (the inclination angle between the disk and the orbital plane is 35◦), where the maximum ram pressure NGC 4569 is a bright spiral galaxy (Sb) whose projected angu- occurred ∼300 Myr ago, are consistent with the Hi observations . ◦ = lar distance to the Virgo Cluster center (M 87) is only 1 7 (Vollmer et al. 2004). 1 0.5Mpc . Because of its brightness and large diameter (D25 = ff  Tschöke et al. (2001) discovered a di use extraplanar re- 9.5 = 47 kpc), Stauffer et al. (1986) have questioned its α i gion of X-ray and H emission to the west of the galactic disk. cluster membership. NGC 4569 has about one tenth the H con- This was the first evidence of a direct connection between the tent of a field galaxy of the same morphological type and the hot X-ray gas and the Hα emission at scales of 10 kpc. This ion- same size (Giovanelli & Haynes 1983). It shows a strongly −1 i ized gas is flowing out from the disk at a velocity of 120 km s truncated H disk (Cayatte et al. 1990), most probably a sig- (Bomans et al. 2005). The most probable source of this outflow nature of strong stripping by the intracluster medium which is a central starburst (Barth & Shields 2000; Tschöke et al. pervades the Virgo cluster (Cayatte et al. 1994). Tschöke et al. 2001). Present-day AGN activity is ruled out due to the lack − (2001) did not find any soft X-ray emission (0.1 0.4 keV) from of a compact point source in the ASCA hard band (Tschöke the northern half of the disk, whereas it is pronounced in the et al. 2001), a missing radio continuum point source (Neff & southern disk (cf. Fig. 1). These findings are consistent with a Hutchings 1992; Hummel et al. 1987), and spectral synthesis ram pressure scenario where the galaxy is moving to the north- analysis (Barth & Shields 2000). Gabel & Bruhweiler (2002) α east through the intracluster medium. The H emission distri- dated the nuclear starburst in the inner ∼30 pc to 5−6Myr bution is, as is Hi, sharply truncated at 30% of the optical ra- i based on optical and UV HST spectra. In addition, Keel (1996) dius (Koopmann et al. 2001). In addition, an anomalous H and found a more extended (∼300 pc) population of A-type super- Hα arm is detected to the west of the galactic disk. Simulated ∼ i giants whose age is greater than 15 Myr. Thus the nucleus of H gas distributions and velocity fields of a more edge-on ram NGC 4569 contains at least two distinct young stellar popula- tions: a very young UV core (5−6 Myr) and a spatially more 1 We use a distance to the Virgo cluster of D = 17 Mpc. extended region dominated by A supergiants.

Article published by EDP Sciences and available at http://www.edpsciences.org/aa or http://dx.doi.org/10.1051/0004-6361:20053819 466 K. T. Chy˙zy et al.: Large-scale magnetized outflows from the Virgo Cluster spiral NGC 4569

In this work we use radio polarimetry to study the gas from both horns, using the “software beam-switching” tech- outflows from NGC 4569. Total power emission and polarized nique (Morsi & Reich 1986). This was followed by restoration radio continuum emission represent very sensitive tracers of of total intensities (Emerson et al. 1979), map transformation flows of diffuse magnetized gas, often unnoticed in the Hi and to RA/Dec coordinates and spatial frequency-weighted combi- Hα lines or in X-rays (Beck et al. 1999; Soida et al. 2002). nation leading to the final Stokes I map. While the polarization allows one to study the magnetic field Because of azimuthal mounting of the radio telescope the Q in the plane of the sky, the Faraday rotation measures the sign and U data at both frequencies were corrected for the rotation and strength of the magnetic fields along the line-of-sight. The of the telescope reference frame of polarization with respect knowledge of the spectral index variations across radio contin- to the sky. At 8.35 GHz the distributions of Stokes parame- uum structures allows us to trace the history of relativistic elec- ters were combined into final Q and U maps using the same tron population transported with the gas and magnetic fields. technique as for total power data. At 4.85 GHz the Stokes U In the absence of an in-situ acceleration mechanism the rel- and Q data for each coverage from both horns were averaged, ativistic electrons lose their energy via synchrotron emission. then rotated to the RA/Dec frame and combined into final Q Since electrons with higher energies lose their energy more and U maps. A digital filtering process that removes spatial rapidly, the radio spectrum steepens with time (electron aging). frequencies corresponding to noisy structures smaller than the To obtain maps of Faraday rotation measures and spectral index telescope beamwidth was applied to final maps of all the Stokes we performed observations at two frequencies: 4.85 GHz and parameters at both frequencies. The Q and U maps were fi- 8.35 GHz. Additionally we used the Hi continuum at 1.4 GHz nally converted into maps of polarized intensity, accounted for obtained by Vollmer et al. (2004). The observations and the the “positive noise bias” in the way described by Wardle & data reduction are presented in Sect. 2. The results are shown Kronberg (1974). We also computed the distributions of appar- in Sect. 3 and discussed in Sect. 4. We give our conclusions in ent (i.e. uncorrected for Faraday rotation – small in our case) Sect. 5. polarization B-vectors, defined as arctan(U/Q) + 90◦. Instrumental polarization of the Effelsberg radio telescope forms specific “butterfly patterns” in Q and U maps at the level 2. Observations and data reduction of about 1% of an unpolarized signal (see Klein et al. 1982 Observations at 8.35 GHz and with a 1.1 GHz receiver band- for examples). They yield an almost axisymmetric structure in width were made using the single-horn receiver at the sec- polarized intensity and in orientation of polarization vectors, ondary focus of the 100-m Effelsberg radio telescope2.The constant in the reference frame of the telescope. Corrections galaxy was observed by making the maps (called coverages), for the azimuthal mounting makes the vectors of instrumental scanned alternatively in RA and Dec. A total of 22 cover- polarization rotate by the parallactic angle. In the case of obser- ages was obtained. Four data channels were recorded. The first vations at a wide range of parallactic angles (which is our case) two channels contain total power signals. The correlations of the superposition of spurious polarization patterns with vari- the left- and right-handed circular polarization signals (giv- ous angles reduces the influence of instrumental polarization ing Stokes Q and U) are recorded in the other two channels. by several times. With the unpolarized intensities discussed be- At 4.85 GHz we used the two-horn system in the secondary low any instrumental polarization is well below the noise. focus of the Effelsberg telescope with a receiver of 0.5 GHz The rms noise levels in total and polarized intensity at bandwidth. Four data channels (two Stokes I as well as Q 8.35 GHz are 0.25 mJy/b.a. and 0.045 mJy/b.a., respectively. / and U – see above) were recorded for each horn. We obtained At 4.85 GHz the corresponding noise levels are 0.4 mJy b.a. in / 10 azimuth-elevation coverages of NGC 4569. total intensity and 0.09 mJy b.a. in polarized intensity. At 1.4 GHz we used the continuum residuals after sub- The telescope pointing was checked at time intervals of i about 2 h by making cross-scans of nearby strong point sources. tracting the H line signal from the observations by Vollmer The flux density scale was calibrated by mapping the highly po- et al. (2004). Because of a low sensitivity (bandwidth of only  ×  larized source 3C 286 and computing its total power flux den- 3 MHz) a convolution to the beamwidth of 65 50 was ap- sities of 4.47 Jy at 8.35 GHz and 7.44 Jy at 4.85 GHz using the plied to show the low-surface brightness structure. No polariza- formulae of Baars et al. (1977). The same calibration factors tion information was recorded at this frequency. The rms noise / were used for total power and polarized intensity, which yields level in this map is 0.3 mJy b.a. a mean degree of polarization of 3C 286 of 11% at 8.35 GHz and 10.5% at 4.85 GHz, in agreement with published values 3. Results (Tabara & Inoue 1980). The data reduction was performed using the NOD2 data 3.1. The global distribution of radio emission reduction package (Haslam 1974). At 8.35 GHz all the total at 4.85 GHz power coverages were combined into the final Stokes I map us- ing the spatial frequency weighting method (Emerson & Gräve In contrast to the X-ray emission (Tschöke et al. 2001) the total 1988). At 4.85 GHz we combined the total power information power brightness at 4.85 GHz shows large extensions on both sides of the galactic disk (Fig. 1). These large structures were 2 The 100-m telescope at Effelsberg is operated by the Max- totally unexpected. In the eastern lobe the radio surface bright-  Planck-Institut für Radioastronomie (MPIfR) on behalf of the ness initially decreases with distance up to about 6 (∼30 kpc) Max-Planck-Gesellschaft. and then quickly drops. The western lobe is coincident with K. T. Chy˙zy et al.: Large-scale magnetized outflows from the Virgo Cluster spiral NGC 4569 467

NGC 4569 4.85 GHz Total power + Pol. int. B-vectors NGC 4569 4.85 GHz Pol. int + Pol. deg. B-vectors 13 18 13 18

16 16

14 14

12 12

10 10

08 08 DECLINATION (J2000) DECLINATION (J2000)

06 06

04 04

02 02 12 37 15 00 36 45 30 12 37 15 00 36 45 30 RIGHT ASCENSION (J2000) RIGHT ASCENSION (J2000) Fig. 1. The total power map of NGC 4569 at 4.85 GHz with superim- Fig. 2. The contour map of polarized intensity of NGC 4569 at posed B-vectors of polarized intensity, overlaid upon the blue image 4.85 GHz with superimposed B-vectors of polarization degree, over- from the DSS. The contour levels are (3, 5, 8, 12, 20, 30, 50, 80) × 0.4 laid upon the Hα image taken from GOLDMine database (Gavazzi (rms noise level) mJy/b.a. The polarization vector of 1 corresponds et al. 2003). The contour levels are (3, 5, 8, 12, 20, 30, 50, 80) × 0.09 to a polarized intensity of 0.5 mJy/b.a. The angular resolution is 2.5. (rms noise level) mJy/b.a. The polarization vector of 1 corresponds to the polarization degree of 20%. The angular resolution is 2.5. the X-ray emission and shows a more gradual decrease in sur-  face brightness. Its extent is smaller than the eastern lobe (4 ∼ lobe has a similar shape in total power. However, it is some- 20 kpc). At the position of the IC 3583, bright what weaker at 8.35 GHz than the eastern region. On this im- total power emission is seen. age IC 3583 is separated from NGC 4569. The disk of NGC 4569 and both lobes show significant po- Our observations at 8.35 GHz show that the polarized emis- larization at 4.85 GHz (Figs. 1, 2). In the disk the B-vectors sion on the western and NW disk side is concentrated in three ◦ (i.e. E-vectors rotated by 90 ) are parallel to the disk plane, separate features (Fig. 4): the most northern polarized peak is but in the extended radio structures the B-vectors are mostly associated with IC 3583 and the ridge of polarized emission perpendicular to the disk. The mean polarization degree in the seen at 4.85 GHz west of the disk appears to be composed disk is 5% increasing to 15% in the extraplanar radio-emitting of two regions at the distance of about 3 from the centre of regions. The radio emission from IC 3583 is polarized as well, NGC 4569. The fact that they apparently coincide with regions with a degree of polarization of about 12%. The B-vectors are showing little total power emission is due to a five times higher oriented towards NGC 4569. noise level in total power channels (due to confusion with back- The polarized intensity in the eastern lobe forms a struc- ground sources and unpolarized atmospheric noise). We can ture elongated perpendicularly to the disk, only barely resolved thus detect significant polarization even where the total power along its minor axis (Fig. 2). In contrast, the polarized emis- signal is weak and very noisy. Both polarization peaks west of sion on the western side extends smoothly along the whole the disk show B-vectors perpendicular to the disk plane. disk edge. Even taking into account the beam-smeared contri- The map at 1.4 GHz made from the line-free channels of bution from IC 3583 leaves the polarization considerably more the Hi observations by Vollmer et al. (2004) has the highest an- extended along the disk on the western than on the eastern disk gular resolution of our data, but contains no polarization chan- side. nels. The eastern lobe seems again to be a detached structure and the thin radio bridge connecting it to the disk of NGC 4569 3.2. Details of the radio structure at 8.35 GHz is also confirmed (Fig. 5). The western extension has a more and 1.4 GHz complex structure and seems to be connected to the disk. In its southern part a relatively narrow ridge of total power emis- Our maps at 8.35 GHz with an angular resolution of 1.5 sion extends away from the disk. It coincides roughly with the show the details of the total power and polarization structure southern polarized extension seen west of the disk at 8.35 GHz of NGC 4569 (Fig. 3). The eastern lobe appears to be a de- (Fig. 4). Bright radio emission from IC 3583, which might be tached region with some hints of a radio bridge connecting it connected to that of NGC 4569 via another putative faint ra- to the disk. Only its northern part is polarized by some 10% dio bridge, is also visible in Figs. 1 and 3. The possible in- with B-vectors almost perpendicular to the disk. The western teractions between the galaxies has already been discussed by 468 K. T. Chy˙zy et al.: Large-scale magnetized outflows from the Virgo Cluster spiral NGC 4569

NGC 4569 8.35 GHz Total power + Pol. int. B-vectors NGC 4569 8.35 GHz Pol. Int. + Pol. deg. B-vectors 13 18 13 18

16 16

14 14

12 12

10 10

08 08 DECLINATION (J2000) DECLINATION (J2000)

06 06

04 04

02 02 12 37 15 00 36 45 30 12 37 15 00 36 45 30 RIGHT ASCENSION (J2000) RIGHT ASCENSION (J2000) Fig. 3. The total power map of NGC 4569 at 8.35 GHz with superim- Fig. 4. The contour map of polarized intensity of NGC 4569 at posed B-vectors of polarized intensity, overlaid upon the blue image 8.35 GHz with superimposed B-vectors of polarization degree, over- from the DSS. The contour levels are (3, 5, 8, 12, 20, 30, 50, 80) × 0.25 laid upon the Hα image (from GOLDMine database, Gavazzi et al. (rms noise level) mJy/b.a. The polarization vector of 1 corresponds 2003). The contour levels are (3, 5, 8, 12, 20, 30, 50, 80) × 0.045  to a polarized intensity of 0.5 mJy/b.a. The angular resolution is 1.5. (rms noise level) mJy/b.a. The polarization vector of 1 corresponds to a polarization degree of 20%. The angular resolution is 1.5.

Tschöke et al. (2001) mentioning the faint southern Hα spur, seen in the Hα+NII image of IC 3583, pointing towards NGC 4569. This is confirmed by a new Fabry Perot Hα image flatter spectral index, cannot be excluded, but high resolution and velocity field obtained for these galaxies (Chemin et al. radio data would be needed to clarify this. 2005). On the other hand, the region of flat spectrum (∼−0.6) in the eastern lobe is certainly real. The slope of about −0.6is only slightly steeper than the injection spectrum of relativistic 4. Discussion electrons in a strong non-magnetic shock in a non-relativistic gas with compression ratio of 4 (Beck & Krause 2005). Since 4.1. Spectral index map no tracers of ionized gas are found in the existing Hα data in Our results clearly indicate the presence of an unexpected bipo- this region, an increased thermal fraction is unlikely, thus in- lar outflow from the inner disk of NGC 4569. To determine the situ electron acceleration in a large-scale shock must take place. history of relativistic electrons in the extended lobes we com- α puted the distribution of the spectral index α (S ν ∝ ν )be- At 8.35 GHz only 70% of the total flux density comes from tween 8.35 GHz and 4.85 GHz over the whole source structure the disk, the remaining 30% being emitted by the extended (Fig. 6). The total power maps at both frequencies were con- structures. Similar fractions are found at 1.49 GHz. Thus, a volved to a common beamwidth of 2.5.Thediskhasarather substantial amount of magnetic and cosmic ray energy is ex- steep spectrum with a slope of about −1.0. The diffuse west- pelled from the galaxy to the intergalactic space. Such verti- ern lobe has an even steeper spectrum with a slope steeper cal nuclear outflows are known to exist in some disk galax- than −1.3 ÷−1.4. The steepening of the spectral index is due ies with strong nuclear starbursts or AGN activity, like M 82 to the energy loss of the relativistic electrons via synchrotron (Reuter et al. 1994), the Circinus Galaxy (Elmouttie et al. 1998) emission. Since electrons with higher energies lose their energy or NGC 4258 (Krause et al. 1996). All these objects have ex- more rapidly, the spectrum steepens if no re-acceleration of the tremely powerful central sources, dominating their radio emis- electrons takes place. Thus the observed steepening of the spec- sion. In NGC 4569 the data at 1.49 GHz processed with the tral index in the western lobe is due to the aging of the relativis- highest possible resolution (beamwidth of 37 × 20 = 3 kpc × tic electrons. Apparent flattenings at the most extreme western 1.6 kpc) show that the unresolved central source comprises no edge and at the northern disk boundary are found in regions more than 15 mJy, thus only some 10% of the total flux density. of very weak signal and may be artifacts of local background High resolution 6 cm and 20 cm data revealed a flux density of variations (a deviation of 1.5 rms value in the 8.35 GHz map 1.5 mJy at 6 cm and 10 mJy at 20 cm in the inner 4 = 330 pc may explain the observed flattening). Acceleration of the cos- (Neff & Hutchings 1992; Hummel et al. 1987). According to mic rays in the southern part of the western lobe, with slightly Gabel & Bruhweiler (2002) half of the flux density at 6 cm is K. T. Chy˙zy et al.: Large-scale magnetized outflows from the Virgo Cluster spiral NGC 4569 469

NGC 4569 1.42 GHz Total power + 8.35 GHz B-vectors NGC 4569 Spectral index 4.85/8.35 GHz -1.5 -1.0 -0.5 13 16

14 13 16

12 14

10 12 DECLINATION (J2000)

08

DECLINATION (J2000) 10

06 08

12 37 15 10 05 00 36 55 50 45 40 35 30 RIGHT ASCENSION (J2000) Fig. 5. Combined image showing the distribution of total power emis- 06 sion at 1.49 GHz (contours and greyscale) and orientations of the 12 37 15 10 05 00 36 55 50 45 40 35 30 RIGHT ASCENSION (J2000) B-vectors at 8.35 GHz. The total power map was made from contin- uum residuals left after subtraction of the Hi signal from the observa- Fig. 6. The distribution of spectral index in NGC 4569 between 4.85 tions of Vollmer et al. (2004). The B-vectors are from the observations and 8.35 GHz from our Effelsberg observations. All the data are con- with the Effelsberg radio telescope. The contour levels are 1, 1.5, 2, volved to a common beamwidth of 2.5. The dashed contours of spec- 2.5, 3, 3.5, 5, 10, 20, 40 mJy/b.a. The rms noise level is 0.3 mJy/b.a. tral index are −1.40, −1.20, −1, −0.800, −0.600. Thick solid con- The beamwidths of the total power and polarization maps are shown tours delineate the brightest features from the full-resolution map at in the left and right lower corner, respectively. The angular resolution 8.35 GHz. is 65 × 50.

4.2.1. The radio geometry due to thermal electrons. They suggest that the rest of the flux density is due to 100−200 supernova remnants. A very surprising aspect of the radio lobes is their relative sym- The eastern lobe has some characteristics of a hot spot at metry. NGC 4569 is moving rapidly within the hot intracluster the terminal point of the jet expelled from a possible AGN in medium. According to Vollmer et al. (2004) the galaxy passed the centre of NGC 4569: is this evidence for a local cosmic the cluster core ∼300 Myr ago and the current ram pressure −12 −1 −2 ray acceleration and a possible connection (jet) to the disk? is about pram ∼ 2 × 10 gcm s . Moreover, the galaxy is The available data exclude the existence of a present-day AGN moving to the north-east. If the observed radio lobes were due in the center of NGC 4569 (see e.g. Barth & Shields 2000 or to an AGN, one would expect the formation of a head-tail ra- Gabel & Bruhweiler 2002). Instead, NGC 4569 harbors a low- dio galaxy, because AGN jets are very vulnerable against forces ionization nuclear emission-line region (LINER). Thus its nu- perpendicular to the outflow direction, which is the case for a cleus shows a very low activity. Furthermore, the western lobe ram pressure wind. In contrast, in a galactic outflow/superwind does not resemble the classical AGN-expelled structure. It is a a pressure driven bubble is created which expands in all direc- diffuse feature attached to the disk with vertical magnetic fields tions and is thus much more stable against forces parallel to emerging from almost the whole inner disk. The possibility of the galactic disk. We therefore suggest that the radio lobes are an earlier AGN activity is discussed in the next section. not due to an AGN (even now extinct), but due to a starburst- induced galactic outflow/superwind. 4.2. A galactic wind in a ram pressure wind 4.2.2. Galactic outflows In this section we discuss various possible origins of the ob- served radio-bubbles in NGC 4569. We start with geometrical We cannot yet determine reliably if the material in the flow aspects of the radio emission and an AGN scenario. We then leaves the potential of NGC 4569 (which is the defining prop- analyse time scales for galactic outflows and galactic super- erty of a galactic wind) or if some or most of the material re- winds. For galactic superwinds we estimate energy needed to turns to the disk of NGC 4569 (which is typical of a galactic blow up radio lobes and compare it to star formation in the disk outflow). The radial velocity of the Hα emitting gas cone at a and two episodes of nuclear starburst. Also, we discuss the in- radius of 4 kpc is measured to be about 100 km s−1 (Bomans fluence of cluster environments on the development of a ridge et al. 2005). The derived time needed to blow up our radio lobe of radio continuum emission in the southern part of the western (25−30 kpc in size) is then about ∼200 Myr, which is too long lobe. a life-time for the synchrotron electrons which do not show 470 K. T. Chy˙zy et al.: Large-scale magnetized outflows from the Virgo Cluster spiral NGC 4569 gradual spectral steepening. On the other hand it is possible of various dispersions were studied by Keel (1996). Narrow that the magnetic field and cosmic rays are connected to a much Balmer absorption lines and the overall shape of optical spec- hotter phase of the gas forming a superwind-like flow at con- trum indicates that the optical light is dominated by a young siderably higher velocity. starburst particularly rich in A-type supergiants. UV light is more peculiar and could not be modelled by the UV light from a dominant population of A supergiants. It was interpreted as 4.2.3. Superwinds an AGN or an additional unusually compact and extraordi- Typical outflow velocities of galactic superwinds are nary luminous central-star cluster. This last possibility was con- − / ∼700−1000 km s 1 (Heckman et al. 2003). So ∼30 Myr firmed in the detailed study of HST FOC UV data by Gabel are needed to develop the observed spatial extent of the radio & Bruhweiler (2002). Their spectral synthesis and photoion- lobes. In order to balance ram pressure, the inner pressure of ization analyses imply that this central starburst is very recent − × 4 the lobes has to be greater or equal to the external ram pressure (5 6 Myr old) and consist of about 5 10 O and B stars packed in a region of ≈30 pc in size. They also pointed out that the pin ≥ pram. Assuming the minimum energy condition we computed the total pressure of cosmic rays and magnetic fields population of A supergiants discerned by Keel (1996) must be > in the lobes. A mean value inside regions delineated by the older ( 15 Myr) and by a factor of 10 more extended than the level of 1.2 mJy/b.a. at 4.85 GHz is ≈1−1.3 × 10−12 dyn cm−2. UV-bright core. This may mean that its content of OB stars It rises to ≥2 × 10−12 dyn cm−2 when brighter parts of lobes (hence supernova progenitors) could be in the past consider- 5 inside the level of 4 mJy/b.a. are considered. Similar values ably greater than 10 . are obtained when pressure balance conditions are assumed. The central present day starburst is clearly too young These values are comparable in range to the ram pressure (see (5−6 Myr) to be responsible for the huge observed radio lobes Sect. 4.2.1). The joint magnetic and cosmic-ray energy density (timescale of ∼30 Myr). Also the number of OB stars gener- is also of the order of 1.5−2 × 1012 erg cm−3, again similar to ated by the recent starburst is less than the minimum num- the kinetic energy density of the wind. The outflows thus have ber of supernovae II (∼105) needed to drive the observed ra- a good chance of overcoming the ambient gas pressure while dio structures. In contrast to that the timescale of the second, the intergalactic wind can still deform the extended lobes older starburst (>15 Myr) seen now as an abundant popula- (Sect. 4.2.5). tion of A-type supergiants matches the timescale of the ob- To estimate the total energy requirements we assume a served outflow. This makes this old starburst (possessing in cone-like geometry for the outflow with a height of 25 kpc and the past enough OB stars) a better candidate to drive the ob- a diameter of 15 kpc, the total volume occupied by the out- served radio lobes. However, further detailed modelling of the flow is thus V ∼ 8 × 1067 cm3. The minimum energy needed A supergiant-dominated population is needed to explain, to- to overcome the external ram pressure within this region then gether with the OB dominated compact starburst, both the UV 56 is E ∼ pinV ≥ pramV = 10 erg. With a typical supernova II and optical spectra and to constrain the starburst geometry, age 51 total kinetic energy input ESN = 10 erg we find that the to- and energetics, and to model the radio lobe formation. A de- 5 tal number of supernovae driving the outflow is NSN ≥ 10 . tailed stellar population synthesis modeling of a past starburst The mass ejected by the wind-type flow is Mwind = ρwindV, which would match the present-day A-star content and produce where ρwind is the mean gas density within the lobes. Assuming enough OB stars at the time of its youth is required to state −1 ffi a mean outflow velocity of vwind = 700 km s the mean density whether such an event is su cient to energize the observed ρ ∼ / v2 = . × −28 −3 lobes. is wind 2E V wind 5 1 10 gcm and the ejected mass ∼ . × 7 is Mwind 1 6 10 M. The fraction between the gravitational ε /ε ∼ v /v 2 ∼ . and the kinetic energy density is grav in rot wind 0 1, 4.2.5. Radio ridge and ram pressure stripping where we assumed a symmetric and a con- −1 stant rotation velocity of vrot = 250 km s .Thus,asufficiently The horizontal ridge of radio continuum emission visible at strong superwind-type flow can easily overcome the gravita- 1.49 GHz in the southern part of the western lobe of NGC 4569 tional potential of the galaxy and resist ram pressure. (Fig. 5) coincides with a spot of polarized emission (Fig. 4) par- allel to this structure. The western side of the galactic disk is the side closest to the Virgo Cluster centre. In the ram pressure 4.2.4. Disk star formation and the central starbursts scenario of Vollmer et al. (2004) NGC 4569 is moving within Is the star formation activity of NGC 4569 strong enough to ex- the Virgo intracluster medium to the north-east. Consequently, plain such powerful wind-type flows? The disk of NGC 4569 the ISM of NGC 4569 is pushed by ram pressure to the south- is weakly forming stars. Due to the low star formation rate the west (last frame of Fig. 6 of Vollmer et al. 2004). Thus, ram arm-interarm contrast in the disk is low and the galaxy is classi- pressure is not directly responsible for the horizontal ridge of fied as anemic by van den Bergh (1976). Thus the star forming radio continuum emission. Hα disk (e.g. Koopmann et al. 2001) of NGC 4569, truncated We suspect the horizontal ridge of radio continuum emis- by the recent ram pressure stripping event, cannot provide the sion to be outflowing gas of the galactic superwind colliding necessary energy input to drive the observed radio lobes. with the part of the stripped ISM of NGC 4569 which is still We are thus left with central starbursts. IUE ultraviolet close to the galaxy. If this gas is atomic its surface density and optical ground-based spectra of the nucleus of NGC 4254 must be less than 1020 cm−2, because it was not detected in K. T. Chy˙zy et al.: Large-scale magnetized outflows from the Virgo Cluster spiral NGC 4569 471 the VLA data of Vollmer et al. (2004). On the other hand, if and have not yet been observed in a cluster spiral galaxy. The this putative south-western Hi plume is more extended than extraplanar western ridge of unpolarized and polarized radio 7 20−30 kpc, its Hi mass cannot exceed several 10 M,be- continuum emission is also a peculiar feature, which is most cause it would have been detected with the Effelsberg 100-m probably due to the cluster environment. telescope. The collision of outflowing and stripped gas leads The available data exclude present-day and past AGN ac- to compression of the outflowing gas and the magnetic field tivity in the core of NGC 4569. We exclude that the observed contained in it, giving rise to the observed horizontal ridge of radio lobes are due to an extinct AGN, because NGC 4569 un- enhanced total power and polarized radio continuum emission. dergoes relatively strong ram pressure effects (Vollmer et al. In addition, we expect that a shock is formed when the wind 2004) which would lead to a head-tail galaxy. Instead, we hits the relatively dense stripped ISM of NGC 4569. The ob- propose that the observed radio lobes are due to galactic served flattening of the spectral index in the south-western part superwind-like flows induced by a starburst lasting several of the western radio lobe might then be due to in-situ particle 10 Myr. Such an event, which requires 105 supernovae explo- acceleration in this large-scale shock. sions and 1056 erg total input energy, is consistent with a stellar The flat spectral index of the eastern lobe and the im- population dominated by a large number of A supergiants in the plied in-situ acceleration of relativistic electrons there might central region of the galaxy (Keel 1996). This is supported by be due to a large-scale shock driven by the direct impact of ram estimates of the combined magnetic and cosmic-ray pressure pressure on this lobe, whereas the western lobe is protected inside the lobes from our radio data. from the direct impact of ram pressure by the stripped ISM of Our observations give the first evidence that galactic NGC 4569. This shielding might also be partly responsible for superwind-like flows can occur even in a spiral galaxy located the different aspects of the two radio lobes. High resolution ra- near the cluster center (projected distance = 0.5Mpc.)They dio continuum observations are needed to confirm and resolve can expand to distances of several 10 kpc and might finally the distribution of the spectral index in the radio lobes. If our escape the galaxy’s gravitational potential and enrich the intra- scenario is correct we would expect a flatter spectral index in cluster medium with gas, dust and magnetic fields. In addition the direction of the ram pressure wind, i.e. to the north-east in to magnetic fields expelled from galaxies during their interac- the eastern lobe. tions (Chy˙zy & Beck 2004) objects like NGC 4569 may consti- Of course, our scenario has caveats: (i) why is the stripped tute an important source of intergalactic/intracluster magnetic and partly re-accreting gas seen in Fig. 6 of Vollmer et al. fields, thereby relaxing the seed field problem in dynamo theo- (2004) located more to the west of the galaxy center than ries (e.g. Widrow 2002). to the south-west? (ii) Can such gas, if it exists, efficiently Despite this first evidence, we are far from fully under- shield/protect the galactic wind-like flows? Of what type is the standing the nature of the giant outflows in NGC 4569. High stellar population, what its age and to what extent is it responsi- resolution, high sensitivity, multi-frequency radio continuum ble for radio lobes? More sensitive multi-frequency radio polar- observations including polarization will greatly help to test our ization and optical observations of NGC 4569 with high spatial tentative scenario. The polarized radio emission detected from resolution would greatly help to establish a detailed scenario of IC 3583 and the existence of a radio bridge connecting IC 3583 these peculiar gas outflows. and NGC 4569 suggest a possible interaction between the two objects which will have to be addressed, too. 5. Conclusions Acknowledgements. The authors wish to thank colleagues from the Using the Effelsberg radio telescope at 4.85 GHz and 8.35 GHz Max-Planck-Institut für Radioastronomie (MPIfR) in Bonn for their we discovered large scale outflows in the strongly stripped valuable discussions during this work. KCh, MS, and MU are in- Virgo spiral galaxy NGC 4569. We found that: debted to Professor Richard Wielebinski from the MPIfR for the invitations to stay at this institute, where substantial parts of this – NGC 4569 possesses large radio lobes extending up to work were done. This work was supported by a grant from the 24 kpc from the disk, which is unusual for normal spirals Polish Research Committee (KBN), 0249/P03/2001/21. D.J.B. ac- and even more unusual for cluster spirals. knowledges the SFB 591 “Universal Behavior of non-equilibrium – The lobes emit as much as 30% of the total flux density at plasmas”. We have made use of the LEDA and GOLDMine databases. 8.35 GHz. – The eastern lobe shows signatures of in-situ shock-driven cosmic ray acceleration. References – The western lobe looks like a diffuse halo with vertical Baars, J. W. M., Genzel, R., Pauliny-Toth, I. I. K., & Witzel, A. 1977, magnetic fields spread over the whole disk. A&A, 61, 99 – Using the continuum channels at 1.4 GHz from the Hi ob- Barth, A. J., & Shields, J. C. 2000, PASP, 112, 753 servations we suggest that its southern part shows effects of Beck, R., Ehle, M., Shoutenkov, V., Shukurov, A., & Sokoloff,D. gas and magnetic field compression by the ambient intra- 1999, Nature, 397, 324 cluster medium. Beck, R., & Krause, M. 2005, Astr. Nachr., in press Bomans, D. J., Hensler, G., Tschöke, D., Boselli, A., & Napiwotzki, The radio structure of NGC 4569 differs in many respects from R. 2005, A&A, submitted that of normal spiral galaxies: the radio lobes with scales of tens Cayatte, V., van Gorkom, J. H., Balkowski, C., & Kotanyi, C. 1990, of kiloparsecs are very rare phenomena in field spiral galaxies AJ, 100, 604 472 K. T. Chy˙zy et al.: Large-scale magnetized outflows from the Virgo Cluster spiral NGC 4569

Cayatte, V., Kotanyi, C., Balkowski, C., & van Gorkom, J. H. 1994, Koopmann, R. A., Kenney, J. D. P., & Young, J. 2001, ApJS, 135, 125 AJ, 107, 1003 Krause, M., & Löhr, A. 2004, A&A, 420, 115 Chemin, L., et al. 2005, A&A, submitted Morsi, H. W., & Reich, W. 1986, A&A, 163, 313 Chy˙zy, K. T., & Beck, R. 2004, A&A, 417, 541 Neff, S. G., & Hutchings, J. B. 1992, AJ, 103, 1746 Elmouttie, M., Haynes, R. F., Jones, K. L., Sadle, E. M., & Ehle, M. Reuter, H.-P., Klein, U., Lesch, H., Wielebinski, R., & Kronberg, P. P. 1998, MNRAS, 297, 1202 1994, A&A, 282, 724 Emerson, D. T., Klein, U., & Haslam, C. G. T. 1979, A&A, 76, 92 Soida, M., Beck, R., Urbanik, M., & Braine, J. 2002, A&A, 394, 47 Emerson, D. T., & Gräve, R. 1988, A&A, 190, 353 Stauffer, J. R., Kenney, J. D., & Young, J. S. 1986, AJ, 91, 1286 Gabel, J. R., & Bruhweiler, F. C. 2002, AJ, 124, 737 Tabara, H., & Inoue, M. 1980, A&AS, 39, 379 Gavazzi, G., Bodelli, A., Donati, A., Franzetti, P., & Scodeggio, M. Tschöke, D., Bomans, D. J., Hensler, G., & Junkes, N. 2001, A&A, 2003, AJ, 400, 451 380, 40 Giovanelli, R., & Haynes, M. P. 1983, AJ, 88, 881 van den Bergh, S. 1976, ApJ, 206, 883 Haslam, C. G. T. 1974, A&AS, 15, 333 Vollmer, B., Cayatte, V., Balkowski, C., & Duschl, W. J. 2001, ApJ, Heckman, T. M. 2003, RMxAC, 17, 47 561, 708 Hummel,E.,vanderHulst,J.M.,Keel,W.C.,&Kennicutt,R.C.,Jr. Vollmer, B., Balkowski, C., Cayatte, V., van Driel, W., & Huchtmeier, 1987, A&AS, 70, 517 W. 2004, A&A, 419, 35 Keel, W. C. 1996, PASP, 108, 917 Wardle, J. F. C., & Kronberg, P. P. 1974, ApJ, 194, 249 Klein, U., Beck, R., Buczilowski, U. R., & Wielebinski, R. 1982, Widrow, L. 2002, Rev. Mod. Phys., 74, 775 A&A, 108, 176